[EM] Fw: borda count
Warren Schudy
wschudy at WPI.EDU
Sun Nov 7 20:20:23 PST 2004
On Sun, 7 Nov 2004, Steve Eppley wrote:
> If James used the term Borda to refer to any scoring rule,
> which is a possible interpretation of his premise about
> "no matter how" the points are allocated to successive
> preferences, then his supposition is wrong. Plurality
> rule is a scoring rule where S1 = 1 and S2=S3=...=Sn=0,
> and it always elects a candidate ranked first by an
> absolute majority, if such a candidate exists. Any scoring
> rule such that S1 >= 2 * (S2 + S3 + ... + Sn) has that
> property. (So do the scoring rules where n = 2 and
> S1 > S2.)
Maybe I shouldn't beat a dead horse (voting method?), but that condition
on scoring rules electing a 1st-by-majority is wrong. As a
counter-example, consider 3 candidates with S1=2,S2=1,S3=0. This satisfies
the condition on S1 >= 2*(S2+...Sn), but with the following ballots it
doesn't elect the 1st-choice majority:
51: A>B>C
49: B>C>A
A: 102 B: 149 C: 49
Another way to see that its wrong: scoring rules produce the same result
if one replaces S_i by M*S_i+L, M>0, but S1>=2*(S2+...SN) changes when
such a transformation is made to the S_i.
I think that plurality is the only scoring rule that also elects a
candidate first-choice by a majority. Consider S1=1, S2=s, S3=0, and
fraction 0.5+f voting A>B>C, 0.5-f voting B>C>A.
A: 0.5+f
B: 0.5-f+s*(f+0.5)
A-B = 2f-s*(f+0.5)
If f < 1/(4/s-2), then A loses despite getting more than half the votes.
This shows that there is no scoring rule that ensures this form of
majority rule for the N=3 case other than plurality. (Replacing S_i with
L*S_i+M for L,M independant of i, L>0, does not affect the outcome, so
setting S1=1,Sn=0 does not lose any generality other than the silly rule
S1=S2=S3)
For N>3, the same result holds: use the same example, with C replaced by
N-2 clones of C.
-wjs
/-----------------------------------------\
| Warren Schudy |
| WPI Class of 2005 |
| Physics and computer science major |
| AIM: WJSchudy email: wschudy at wpi.edu |
| http://users.wpi.edu/~wschudy/ |
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